1. Field of the Invention
The present invention relates to the purification of impure phosphoric acid, and, more especially, relates to the sulfate ion removal from a raw acid to obtain a food grade phosphate having a sulfate ion content, by weight, of less than 50 ppm based upon the weight of the P.sub.2 O.sub.5.
2. Description of the Prior Art
It is known to this art to effect production of purified phosphoric acid or purified phosphate by [1] the strong acid attack, e.g., sulfuric acid attack, upon phosphate rock, [2] extraction of the raw liquor thus obtained by means of an organic phase for the extraction of the phosphoric acid, said organic phase typically including a C.sub.4 to C.sub.8 alkyl phosphate or aliphatic alcohol, [3] followed by the re-extraction of the organic phase with either water or an aqueous solution, optionally after washing with water. Typically, an excess of sulfuric acid is utilized for the attack upon the phosphoric rock and, in addition, it may be advisable to extract the phosphoric acid from the raw liquor in the organic phase by adding sulfuric acid to the extraction installation, as described in U.S. Pat. No. 3,607,029, in order to improve the phase transfer of the phosphoric acid and, thus, the yield of extraction. If this be done, however, the organic phase containing the phosphoric acid also contains a not insignificant amount of sulfuric acid. Subsequently, the noted organic phase is usually washed with water, ultimately followed by re-extraction of the phosphoric acid with water or an aqueous alkaline phase; thus is outlined a procedure whereby a phosphoric acid or a substantially pure phosphate solution, nonetheless comprising a not insignificant amount of sulfate ions, is obtained [compare also U.S. Pat. No. 3,767,769]. Consequently, utilizing those techniques set forth in the aforesaid two patents, an inadequate purity results. It would, of course, be possible to enhance the purity of the resulting product by increasing the number of washing stages, but this would result in the disadvantage of considerably increasing the size of the wash installation and the costs of manufacture.
It too has been proposed to rid the organic phase of extraction of sulfate ion, containing the phosphoric and sulfuric acids, by treating same with an aqueous solution containing phosphate and alkaline earth metal (Ca, Ba, Sr) ions, and this is the reason the sulfate ions in the aqueous phase are extracted and precipitated in the form of alkaline earth sulfates [compare U.S. Pat. No. 4,118,462, assigned to the assignee hereof].
Nonetheless, for several years, phosphoric acid or phosphates having a very low content of sulfate ion, in most cases less than 50 ppm with respect to the P.sub.2 O.sub.5, have been required for human nutrition purposes.